1. Field of the Invention
The present invention relates to a TDMA digital mobile communication system such as a Personal Handyphone System (PHS).
2. Description of the Related Art
In TDMA (Time Division Multiple Access) digital mobile communication systems of the prior art, neighboring radio base stations each use a frequency selected by each radio base station for transmitting and receiving, and the timing of transmission and reception used by these radio base stations is mutually asynchronous. As a result, a plurality of radio base stations will sometimes transmit at the same frequency and at the same timing, thereby giving rise to frequency interference. When such instances of frequency interference occur, transmission must be switched to another time slot, to another frequency, or to another radio base station, or alternatively, transmission must be temporarily halted in order to avoid this frequency interference.
FIG. 1 is provided for illustrating matching and non-matching between synchronous timing between radio base stations. Here, (a), (b), and (c) each show transmission and reception time slots T1, T2, T3 . . . and R1, R2, R3 . . . for radio base stations A, B, and C, respectively. All of the frequencies used at this time are f.sub.1, and the timing at radio base stations A and B coincide entirely while the timing at radio base stations B and C are shifted with respect to each other. The mobile stations that communicate with base stations A, B, and C are further synchronized to the timing of the corresponding base station.
In such a case, when radio base stations B and C are adjacent to each other, radio base station B transmits in time slot T3, and radio base station C transmits in time slot T2, a time slot distinct from that of radio base station B. In this case, radio base stations B and C transmit asynchronously, and therefore, may transmit at identical or overlapping timing even though different time slots are employed, thereby giving rise to frequency interference on the side of the corresponding mobile stations. However, frequency interference will not occur, even between neighboring slots, if synchronization is established as for radio base station A and radio base station B. In other words, at the same frequency, effective use is enabled for any time slot.
The prior art suffered from the drawback that, because the timing of transmitted and received radio waves was asynchronous for each radio base station, the occurrence of frequency interference necessitated switching to another slot or switching to another frequency, thereby delaying transmission and reception and preventing effective communication.
To deal with the above-described drawback, measures have been taken such that each radio base station is synchronized to a prescribed timing. For example, in Japanese Patent Laid-open No. 224325/91, one control station measures in advance the delay time of signal transfer to each radio base station, and then coordinates such that timing pulses transmitted from the control station to each radio base station are simultaneously received at each radio base station and a frame generation circuit in each radio base station is reset. As a result, each radio base station transmits and receives at the same frame timing and adjacent radio base stations can transmit and receive the same frequency channel in the adjacent time slots.
In the present invention, instead of sending the timing pulses, synchronizing signal data are sent from a synchronized timing control station to base stations, where the received data are compared with the same synchronizing signal data held by the base stations.